The following account of the prior art relates to one of the areas of application of the present application, hearing aids.
In hearing aids, signals are analyzed and processed in frequency bands. In order to reduce the power consumption, the many frequency bands (often uniformly distributed on the frequency axis) are combined into fewer channels and the processing is done in those combined bands. The result of the processing in each channel may e.g. be a gain, which is redistributed into the many frequency bands, by being multiplied to the signal values of each frequency band and finally synthesized into an output signal.
US 2006/0159285 A1 describes a hearing aid wherein the number of channels in which the signal is processed can be (dynamically) changed, e.g. depending on the acoustic environment or a particular program selection.
U.S. Pat. No. 6,240,192 describes a filter bank structure having the option of varying the number of bands (bandwidth, overlap or non-overlap, etc.).
U.S. Pat. No. 5,597,380 describes a cochlear implant type hearing aid where a number of processing channels is selected from a larger number of input channels in order to provide a balance between the quantity and resolution of information in the frequency domain, and resolution in the time domain.
US 2006/013422 A1 deals with a cochlear implant comprising two types of analysis filter banks for processing different frequency ranges of an input signal differently. Further the number of channels may be selected (e.g. to match the number of electrodes in a particular cochlear implant device). In an embodiment, the number of channels may be increased to enhance any region of the spectrum where finer spectral detail might be required.
U.S. Pat. No. 6,311,153 describes an audio signal compression apparatus comprising frequency warping, whereby a low frequency band, which is auditorily important, can be analyzed with a higher frequency resolution as compared with a high frequency band, whereby efficient signal compression utilizing human auditory characteristics is realized.
US 2009/017784 A1 describes a method of adaptively processing an input signal, the method comprising passing the input signal through an adaptive warped time domain filter to produce an output signal. The scheme has the advantage of flexibility in allowing more selective or non-uniform resolution filters in the filter-bank, for example to mimic the Bark scale, or to reflect critical bands in human hearing.
EP 2 190 217 A1 describes a method of reducing feedback in a hearing aid by multiplying a plurality of upper frequency bands by a random phase.
US 2004/258249 deals with a directional microphone system and the mixing of frequency bands from different microphones.
US 2007/076910 A1 describes cross-over of selected frequency bands from one hearing instrument to another in a binaural hearing aid system.